Hello Gang! FNG Chiming in for the first time. And do I have a mystery I need help solving.
So my ride is a 1956 F100. The Drive train consists of a Reman 1977 Ford 400(D7TE) with a 77 C-6 Tranny and a 65 Bronco 9 inch rear end.

500 miles ago I threw a couple rods and cracked a head, so I replaced with the Australian 351 heads (no smog crap). Went over the top end with new rods lifters gaskets water pump etc.
A week ago on the hiway (running great at 70 mph) the small gear on my camshaft shredded after I put in a new TSP HEI distributor in it. it lasted about 400 iles and then just ground down to a nub sending gear teeth down thru the crank into the pan. The cam is toast but this bastard needs to be gone through.
I now also have a significant amount of coolant in my oil pan. (about 2 cups) So I pulled out the motor broke it all down to the block.
I can find nothing in the way of a blown head gasket, intake gasket, visual crack anywhere. When the heads came off it all looked clean with no sign of coolant in the piston bores. I have read every thread out there and all I can think of is there must be an internal crack somewhere inside the valleys of the block. Im Baffled. Ill take this thing to the machine shop for magnaflux-ing but I just don't dare put it back together until I find the damn culprit. Help me out!

Well the block went to the machine shop today. It will be gone through and the machinist says if its cracked he will find it. I removed the harmonic balancer and timing chain cover and although it hadn't been removed for probably 15 years, neither myself of the machinist could really see any evidence of that. But we wont rule it out. Thanks a million for responding.

We the block is not cracked. Came back with a clean bill of health. it was .40 over so will bore it out to .60. There is no tell tale signs what caused that coolant in the oil. Only clue is it was perfectly separated when it came out. No chocolate milk foam at all.
Polish the crank, put a better cam in it and build it back up. Then get it balanced about $1500 total. Can only assume at this point the water went down behind the timing chain cover. Mystery!